1. Field of the Invention
The present invention relates generally to an autodyne receiver, and more particularly to a tuning circuit incorporated in the autodyne receiver. Still more particularly, this invention relates to a tuning circuit for an autodyne receiver which includes a negative resistive impedance converter to cancel the resistive impedance of the tuning circuit to improve the tuning characteristics of the receiver.
2. Description of the Prior Art
In a superheterodyne receiver, it is possible to improve the selectivity by reducing the intermediate frequency of received signals. On the other hand, since the superheterodyne receiver is subjected to image interference, the lower the intermediate frequency, the greater the image interference. In addition, non-uniform sensitivity often occurs within the reception band due to tracking error. Further, there exist such drawbacks that the number of necessary circuit elements is great and the adjustment positions are many after the superheterodyne receiver has been assembled.
In contrast with this, autodyne receivers do not suffer from the above problems involved in the superheterodyne receiver. Further, there has been proposed an autodyne receiver such that a positive feedback is applied to a tuning circuit connected at a previous stage of the detector to increase the tuning sharpness Q of the tuning circut, that is, to obtain a higher selectivity and a higher high-frequency gain.
In general, the tuning sharpness Q of a tuning circuit can be expressed as EQU Q=fo/.alpha.f
where fo denotes a center frequency of the band of passed signals of a tuning circuit, and .DELTA.f denotes a bandwidth corresponding to -3 db response from top at the center frequency fo (see FIG. 4). The above expression indicates that in the autodyne receiver, the higher the tuning sharpness Q, the narrower the bandwidth .DELTA.f at the center frequency fo so that the selectivity of the autodyne receiver will be improved. In addition, the high-frequency gain of the receiver can be increased under positive feedback control.
However, there inevitably exists a limit in applying a positive feedback signal to a high frequency signal tuning circuit because oscillation is readily generated. Therefore, it is difficult to obtain a sufficiently high selectivity and a high high-frequency gain simultaneously in a high frequency tuning circuit. Furthermore, it is difficult to apply an automatic gain control (AGC) signal to the tuning circuit because of its lack of gain. On the other hand, the positive feedback control inevitably results in instability in the tuning circuit.
To overcome the above-mentioned problems involved in the autodyne receiver, there has been proposed another autodyne receiver such that a Q multiplier is connected to an antenna tuning circuit to increase the tuning sharpness Q of the antenna circuit to improve the selectivity thereof. In the autodyne receiver of this type, however, there still exist problems in that the design of the tuning circuit is very severe and further oscillation is often generated when the selectivity is increased, thus involving problems similar to those of the conventional autodyne receiver with respect to the selectivity and stability since each is contrary to the other.
Further, in the case where the medium wave band is from 535 to 1605 kHz, the bandwidth .DELTA.f at the highest reception frequency is about three times wider than that at the lowest reception frequency in accordance with the above expression, thus resulting in another problem in that the selectivity changes markedly according to the magnitude of reception frequency.
Thus it is a continuing problem in the art of providing autodyne receivers to improve the stability of the tuning circuit for the receiver to exhibit high selectivity while maintaining stability, increase its tuning sharpness, and exhibit uniform sensitivity over the frequency range of interest for the receiver.